1. Field of the Invention
The present invention relates to a liquid crystal display device and a method of fabricating the same.
2. Discussion of the Related Art
Until recently, display devices have typically used cathode-ray tubes (CRTs). Presently, much effort has been made to develop various types of flat panel displays, such as liquid crystal display (LCD) devices, plasma display panels (PDPs), field emission displays, and electro-luminescence displays (ELDs), as substitutes for CRTs. Of these types of flat panel displays types, LCD devices have many advantages, such as high resolution, light weight, thin profile, compact size, and low power supply requirements.
In general, an LCD device includes two substrates that are spaced apart and facing each other with a liquid crystal layer interposed between the two substrates. The two substrates include electrodes that face each other such that a voltage applied between the electrodes induces an electric field across the liquid crystal layer. Alignment of the liquid crystal molecules in the liquid crystal layer changes in accordance with the intensity of the induced electric field into the direction of the induced electric field, thereby changing the light transmissivity of the LCD device. Thus, the LCD device displays images by varying the intensity of the induced electric field in respective pixel regions that make up the LCD device. Presently, fast response and wide viewing angle are needed for LCD devices. An optically compensated birefringence (OCB) mode LCD device has these characteristics.
FIG. 1 is a cross-sectional view of an OCB mode LCD device according to the related art. As shown in FIG. 1, an OCB mode LCD device 1 includes an array substrate, a color filter substrate, and a liquid crystal layer 58 between the two substrates.
In the array substrate, a gate line (not shown) and a data line 40 crossing the each other are formed on a first substrate 30. A thin film transistor Tr is disposed at a crossing of the gate line and the data line 40. A pixel electrode 49 is connected to a drain electrode 46 of the thin film transistor Tr. A first alignment layer 55 is disposed entirely on the substrate 30 having the pixel electrode 49.
In the color filter substrate, a black matrix 12 is disposed on a second substrate 10. A color filter layer 15 including red (R), green (G), and blue (B) color filter patterns 15a, 15b, and 15c is disposed to fill openings of the black matrix 12. A common electrode 18 is disposed on the color filter 15. A second alignment layer 25 is disposed on the common electrode 18.
The liquid crystal layer 58 is provided in a cell gap D between the array substrate and the color filter substrate. The first and second alignment layers 55 and 25 are rubbed in one direction so that liquid crystal molecules 59 in the liquid crystal layer 58 are arranged in the rubbed direction with a pre-tilt angle.
FIGS. 2A to 2C are cross-sectional views illustrating operations of an OCB mode LCD device according to the related art.
As shown in FIG. 2A, liquid crystal molecules 59 of the OCB mode LCD device 1 have an initial alignment state i.e., a splay state below a transition voltage (Vt). The liquid crystal molecules 59 adjacent to the array substrate and the color filter substrate are arranged with a tilt angle of −θ and +θ which are pre-tilt angles. A tilt angle of the liquid crystal molecules 59 are reduced toward a center portion of the liquid crystal layer 58. A tilt angle of the liquid crystal molecules 59 at the center portion is zero degrees.
As shown in FIG. 2B, when a voltage equal to or greater than the transition voltage (Vt) is applied, the liquid crystal molecules 59 transitions to a bend state from the splay state. The liquid crystal molecules 59 adjacent to the array substrate and the color filter substrate are still arranged with a tilt angle of −θ and +θ. A tilt angle increases toward a center portion of the liquid crystal layer 58. A tilt angle of the liquid crystal molecules 59 at the center portion is 90 degrees. If a voltage below the transition voltage is applied or a voltage is not applied, the liquid crystal molecules 59 return to the splay state.
As shown in FIG. 2C, when a voltage to display a black image, i.e., a very high voltage is applied, most of the liquid crystal molecules 59, except for the liquid crystal molecules 59 adjacent to the array substrate and the color filter substrate, have a tilt angel of 90 degrees.
FIG. 3 is a graph illustrating a light transmittance to a voltage applied in an OCB mode LCD device according to the related art.
As shown in FIG. 3, the light transmittance is irregular below a transition voltage (Vt), and the light transmittance is reduced almost linearly at voltages equal to or greater than the transition voltage (Vt). The OCB mode LCD device uses a section where the light transmittance is varied linearly to normally display an image having gray levels. Accordingly, whenever the OCB mode LCD device is driven to normally display an image, a splay state should transition into the bend state at an initial stage. To do this, the related art OCB mode LCD device has a driving circuit that applies a voltage equal to or greater than the transition voltage (Vt) to transition the splay state into the bend state.
The light transmittance is highest at the transition voltage (Vt) so the brightest white image could be displayed at the transition voltage (Vt). However, since the voltage applied may have a deviation, the liquid crystal molecules may transition from the bend state back to the splay state near the transition voltage (Vt). To prevent this abnormal re-transition, a white voltage (Va) higher than the transition voltage (Vt) is used to display a white image. The bend state is more stable at the white voltage (Va) than at the transition voltage (Vt). However, the light transmittance at the white voltage (Va) is reduced (Tt>Ta). Therefore, brightness is reduced, and contrast ratio is also reduced.